Add CryptoApi.getActiveSessionBackupVersion() (#3555)

* stub backupmanager

* Implement `CryptoApi.getActiveSessionBackupVersion`

* Revert unnecessary change

we can do this later, once we have better test coverage

* more test coverage

---------

Co-authored-by: Richard van der Hoff <richard@matrix.org>

src/rust-crypto/backup.ts

@@ -0,0 +1,25 @@
+/*
+Copyright 2023 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+export class RustBackupManager {
+    /**
+     * Get the backup version we are currently backing up to, if any
+     */
+    public async getActiveBackupVersion(): Promise<string | null> {
+        // TODO stub
+        return null;
+    }
+}